Preserve local paths for desktop saves (#5543)

# Summary

- Adds desktop file tracking: local paths are preserved and save buttons
now work as expcted (doing Save/Save As as appropriate)
- Adds logic to track whether files are 'dirty' (they've been modified
by some tool, and not saved to disk yet).
- Improves file state UX (dirty vs saved) and close warnings
- Web behaviour should be unaffected by these changes

## Indicators
Files now have indicators in desktop mode to tell you their state.

### File up-to-date with disk

<img width="318" height="393" alt="image"
src="https://github.com/user-attachments/assets/06325f9a-afd7-4c2f-8a5b-6d11e3093115"
/>

### File modified by a tool but not saved to disk yet

<img width="357" height="385" alt="image"
src="https://github.com/user-attachments/assets/1a7716d9-c6f7-4d13-be0d-c1de6493954b"
/>

### File not tracked on disk

<img width="312" height="379" alt="image"
src="https://github.com/user-attachments/assets/9cffe300-bd9a-4e19-97c7-9b98bebefacc"
/>

# Limitations
- It's a bit weird that we still have files stored in indexeddb in the
app, which are still loadable. We might want to change this behaviour in
the future
- Viewer's Save doesn't persist to disk. I've left that out here because
it'd need a lot of testing to make sure the logic's right with making
sure you can leave the Viewer with applying the changes to the PDF
_without_ saving to disk
- There's no current way to do Save As on a file that has already been
persisted to disk - it's only ever Save. Similarly, there's no way to
duplicate a file.

---------

Co-authored-by: James Brunton <[email protected]>
Co-authored-by: James Brunton <[email protected]>

frontend/src/desktop/components/fileEditor/FileEditorFileName.tsx

@@ -0,0 +1,66 @@
+import React from 'react';
+import { Tooltip } from '@mantine/core';
+import { useTranslation } from 'react-i18next';
+import { StirlingFileStub } from '@app/types/fileContext';
+import { PrivateContent } from '@app/components/shared/PrivateContent';
+
+interface FileEditorFileNameProps {
+  file: StirlingFileStub;
+}
+
+const FileEditorFileName = ({ file }: FileEditorFileNameProps) => {
+  const { t } = useTranslation();
+
+  return (
+    <>
+      <PrivateContent>{file.name}</PrivateContent>
+      {!file.localFilePath && (
+        <Tooltip label={t('fileNotSavedToDisk', 'Not saved to disk')}>
+          <span
+            style={{
+              display: 'inline-block',
+              width: '6px',
+              height: '6px',
+              borderRadius: '50%',
+              backgroundColor: 'var(--mantine-color-red-6)',
+              flexShrink: 0
+            }}
+            aria-label={t('fileNotSavedToDisk', 'Not saved to disk')}
+          />
+        </Tooltip>
+      )}
+      {file.localFilePath && file.isDirty && (
+        <Tooltip label={t('unsavedChanges', 'Unsaved changes')}>
+          <span
+            style={{
+              display: 'inline-block',
+              width: '6px',
+              height: '6px',
+              borderRadius: '50%',
+              backgroundColor: 'var(--mantine-color-yellow-6)',
+              flexShrink: 0
+            }}
+            aria-label={t('unsavedChanges', 'Unsaved changes')}
+          />
+        </Tooltip>
+      )}
+      {file.localFilePath && !file.isDirty && (
+        <Tooltip label={t('fileSavedToDisk', 'File saved to disk')}>
+          <span
+            style={{
+              display: 'inline-block',
+              width: '6px',
+              height: '6px',
+              borderRadius: '50%',
+              backgroundColor: 'var(--mantine-color-green-6)',
+              flexShrink: 0
+            }}
+            aria-label={t('fileSavedToDisk', 'File saved to disk')}
+          />
+        </Tooltip>
+      )}
+    </>
+  );
+};
+
+export default FileEditorFileName;